The present invention relates to internal combustion engines and more particularly, to a method and apparatus for determining the air-to-fuel ratio of an internal combustion engine based upon a measurement of the exhaust gas temperature of the engine.
Approaches for reducing undesired emissions in the exhaust gas of internal combustion engines include: (1) operating the engine with engine operating parameters specifically set to minimize the engine generated undesired emissions measured at the engine exhaust manifold, and (2) employing after-treatment of the engine exhaust gas and adjusting the engine operating parameters to minimize the undesired emissions measured at the tailpipe outlet.
In the case of engines used in part-throttle applications such as automobiles, it is the current practice to employ a three-way catalytic converter for after-treatment of the engine exhaust gas. A three-way catalytic converter operating with engine exhaust gas having a stoichiometric air-to-fuel (A/F) ratio of 14.7 is extremely effective in reducing CO, HC and NOX tailpipe emissions. However, in order to achieve the maximum reduction of undesired tailpipe emissions, the A/F ratio of the engine exhaust gas must be tightly controlled to a value of 14.7.
Switching type exhaust gas oxygen (EGO) sensors mounted in the engine exhaust path of automotive exhaust systems are commonly used to provide an indication of whether the A/F ratio of the engine exhaust gas is above or below the desired exhaust gas A/F ratio of 14.7. Switching type EGOs are sensitive, accurate, inexpensive, rugged and well matched to providing the tightly controlled exhaust gas A/F ratio required by catalytic converters. Automobile emission control systems used with fuel injected internal combustion engines typically employ one or more EGOs in a closed loop control system to regulate the A/F ratio to an average value of 14.7 by adjusting the engine fuel injection period for each cylinder event.
After automotive emissions, the next most serious source of air pollution from internal combustion engines in the United States are the gasoline powered internal combustion engines that power lawn mowers. Accordingly, it is desirable to reduce the undesired emissions from lawn mowers.
Internal combustion engines such as those used in lawn mowers (and also marine vessels) operate under continuous high load conditions. These engines typically operate with a rich A/F ratio (i.e. an A/F ratio substantially less than 14.7) in order to yield maximum power from the engine simultaneously with low engine weight and acceptable cooling of the engine. Engine operation at other than an A/F ratio of 14.7 precludes using a three-way catalytic converter for after-treatment of engine exhaust gas. Accordingly means for reducing the undesired emissions from the engines that power lawn mowers and similar engines must do so without the benefit of a three-way catalytic converter.
Engine cooling is critical for engines operating at full load such as those used to power lawn mowers and marine vessels. The cooling of an internal combustion engine increases as the A/F ratio is decreased below 14.7. However, CO and HC emissions increase rapidly as the A/F ratio decreases below the value of 14.7.
In lawnmower and marine applications, for example, it is common practice to preset the A/F ratio of new engines to a predetermined rich value dictated by the engine power output and the cooling requirements, without the benefit of closed loop A/F ratio control. It is well known that a preset engine A/F ratio gradually drifts to a higher value of A/F ratio as the engine wears. Accordingly, it is common practice to preset the A/F ratio of new engines designed for operating under high load conditions to a value that is lower than is necessary for acceptable engine cooling and power, to ensure that adequate engine cooling will be maintained over the life of the engine. Since the A/F ratio is commonly set lower than necessary for acceptable engine operation during much of the life of the engine, the undesired emissions generated by the engine are higher than they would otherwise be if the A/F ratio could be maintained to be more optimum during the engine lifetime.
If the actual A/F ratio of the engine exhaust gas could be directly measured, engines operating with a rich A/F ratio could be controlled by a closed loop control system to set and hold a higher value of A/F ratio than is now commonly employed, thereby maintaining acceptable operating performance and simultaneously reducing undesired CO and HC engine emissions. However, switching type EGO sensors, as used in automotive applications, are only suitable for measuring the exhaust gas A/F ratio when the A/F ratio is centered on a value of 14.7. The only alternative thus found that is practical for accurately measuring the A/F ratio of an operational internal combustion engine is a universal exhaust gas oxygen (UEGO) sensor. However, the UEGO sensor has been found to be too expensive for applications such as a lawnmower motor, and to be unreliable when exposed to water such as would occur in marine applications. Consequently, engines used to power lawnmowers and marine vessels typically operate without the benefit of closed loop control of the engine exhaust A/F ratio.
A number of investigators have developed methods for computing the temperature of engine exhaust gas and/or the temperature of the catalytic converter in automotive applications. These methods have in common, computing the exhaust gas and the catalytic converter temperatures based on a measurement of the exhaust gas A/F ratio. For example, U.S. Pat. No. 4,656,829 teaches an analytical method of computing the catalytic converter temperature based on the exhaust gas A/F ratio, mass air flow and empirical data characteristic of a specific engine/catalytic converter combination. Similarly, U.S. Pat. No. 5,303,168 discloses a method of computing the engine exhaust temperature based on the A/F ratio, exhaust gas recirculation (EGR) rate, spark timing, the mass air flow and the engine speed. Thus, it has been suggested that there is a predictable relationship between the A/F ratio of an engine and the exhaust gas temperature of an engine.
While it has been suggested that a relationship exists between the engine exhaust A/F ratio and the engine exhaust gas temperature, previously developed models representing the A/F ratio/exhaust gas temperature relationship have all been for part-throttle applications such as used in vehicles employing catalytic converters, in which the engine is operating under stoichiometric conditions. Further, each of the aforementioned models computes the exhaust gas temperature from a measured A/F ratio, and requires, in addition to measuring the A/F ratio, extensive other sensor inputs such as mass air flow. None of these known methods discloses or teaches the reverse process of determining the A/F ratio from a set of engine measurements under non-stoichiometric conditions. Accordingly, there is a need for a means for accurately determining the A/F ratio of internal combustion engines operating with rich A/F ratios under sustained high power conditions such as, for example, the type of engines used on lawnmowers and in marine vessels and which relies exclusively on inexpensive and reliable sensors.
In brief the present invention comprises a method using a computer for determining an air-to-fuel (A/F) ratio of an internal combustion engine, wherein information characteristic of the engine relating the A/F ratio of the engine, an exhaust gas temperature of the engine, a speed of the engine and a parameter related to a load of the engine is previously stored in the computer. The method comprises the steps of: measuring the exhaust gas temperature, the speed, and the parameter related to the load; computing the A/F ratio based on the previously stored information, the measured exhaust gas temperature, the measured speed and the measured parameter related to the load; and outputting a signal representative of the A/F ratio.
The present invention also comprises a system for determining an air-to-fuel (A/F) ratio of an internal combustion engine. The system comprises a memory for storing information characteristic of the engine relating the A/F ratio, the exhaust gas temperature, the speed and a parameter related to the load of the engine; a sensor for measuring the exhaust gas temperature of the engine; a sensor for measuring the speed of the engine; a sensor for measuring the parameter related to the load of the load of the engine; and a computer for determining the A/F ratio from a computation based on the stored engine information, the measured exhaust gas temperature, the measured engine speed and the measured parameter related to the engine load and for outputting a signal representative of the A/F ratio.
The present invention further comprises computer executable software code stored on a computer readable medium for computing an air-to-fuel (A/F) ratio of an internal combustion engine. The software code comprises: information characteristic of the engine relating the A/F ratio, the exhaust gas temperature, the engine speed and the parameter related to the engine load; code responsive to receiving a measurement of the exhaust gas temperature of the engine; code responsive to receiving a measurement of the engine speed; code responsive to receiving a measurement of the parameter related to the engine load; and code for computing the A/F ratio based on the measured exhaust gas temperature, the measured engine speed, the measured parameter related to the engine load; and the information relating the A/F ratio, the engine exhaust gas temperature, the engine speed and the parameter related to the engine load.